It is known to use polymer coatings on various substrate materials. Typically, these coatings are produced using thermally activated polymerization. Heat is used to generate active centers that polymerize the coatings after a monomer has been applied to a substrate as a liquid. However, thermal polymerization requires large amounts of energy, time and expense to run high temperature ovens for extended periods of time.
It is also known to use photopolymerization to produce polymer coatings on substrates. Photopolymerization reactions are chain reactions which generate free radical or cationic active centers. In photopolymerization, energy from UV or visible light is used to polymerize the monomer. Photopolymerization has a number of advantages including savings in energy and high cure rates without the necessity of solvents. Conventional photopolymerization proceeds by free radical polymerization process.
Photopolymerization of coatings on various substrates has not been successful because of problems with oxygen inhibition. Previously known photopolymerization systems generally have used free radical polymerization to generate free radical active centers. The free radical active centers may react with oxygen to produce unreactive peroxides and hydroperoxides. This results in a decrease in the polymerization rate and a reduction in molecular weight of the polymer. The oxygen inhibition may cause free radical polymerizations to exhibit an incomplete cure resulting in deficient coatings. A typical method for overcoming oxygen inhibition is to purge the system with nitrogen in an attempt to displace the oxygen from the monomer. Coatings having multiple layers would require nitrogen purging for each layer also requiring more than one illumination step.
Additionally, photopolymerization may also exhibit deficiencies in curing systems having pigments. Pigments may be used in a coating to provide color or to cover the surface of a substrate. Pigments may inhibit photopolymerization by directly competing with a photoinitiator absorption. Additionally, pigments may interact with light to scatter photons in multiple directions resulting in increased light attenuation for pigmented coatings.
Photopolymerization may be further disadvantaged by requiring line of sight exposure of the substrate surface coated with a monomer composition. Line of sight exposure and even illumination of a substrate surface becomes especially difficult for substrates having shadow areas or portions that cannot be oriented directly in the line of sight of exposing illumination.
Cationic polymerization has advantages over conventional free radical polymerization techniques. For example, free radical polymerization is negatively affected by the inclusion of pigmented materials that absorb the illumination required to initiate polymerization and/or interfere and/or capture free radical monomers present during polymerization. Such radical trapping requires line of sight curing of substrates. Surfaces of a substrate coated with a monomer composition but not exposed to line of sight curing may not undergo curing and/or may be subject to insufficient curing and thereby form defective and/or only partially polymerized coatings.
Further, coatings formed by free radical polymerization are often subject to a high shrinkage rate. High shrinkage of a two-dimensional polymer coating covering a large substrate detrimentally affects the adhesion of the coating on the substrate surface and likewise may form a coating having a higher tendency to form defects.
In contrast to free radical polymerization, cationic polymerization may include an “living” component such that after an initial illumination and/or formation of cationic active centers the polymerization is self-propagating even in the absence of further illumination. Polymerization may therefore propagate and migrate into portions of a substrate surface that are not directly illuminated or only partially illuminated. For example, if a two-dimensional substrate such as an automotive body panel is coated on both outside and inside surfaces with a monomer composition and the thus-coated automotive panel is illuminated on only one side, i.e., the illuminated side in contrast to the shadow side, polymerization initiated at the illuminated side may propagate to the shadow side and/or portions of the automotive body panel which are not otherwise illuminated with the same degree of intensity as those portions that are in direct line of sight of the illuminating radiation.
Subsequent to photoinitiation of the cationic active centers and polymerization of the cationically polymerized monomers, cationic centers may continue to be present in the resultant polymerized coating. Polymerization may therefore continue to an unnecessary degree and/or for an extent of time that is not desirable.
There is therefore a need in the art for an improved method of forming a pigmented coating by photopolymerization of controlled duration. There is also a need in the art for an improved method of applying and curing a coating that eliminates the need for multiple illumination and/or nitrogen purging.